In recent years, there has been a demand for a method of minimizing electric bill and carbon dioxide emissions in a building, where multiple power supply sources including a utility grid and a solar power generator and multiples batteries including a power storage unit installed at the building and a vehicle battery mounted on a vehicle are available. In a power supply system disclosed in US 2009/0192655 corresponding to JP 2008-54439A, power exchanged between a vehicle and a building is managed by taking into account power supply conditions in the building.
In order to minimize electric bill and carbon dioxide emissions, a charging-discharging schedule of the battery needs to be determined by taking into account various factors changing with time. For example, the factors can include power consumed in the building, power generated by the solar power generator, power stored in the battery, a period of time where power can be supplied from the vehicle battery to the building, and an unit electricity price depending on time of day. It is difficult to determine an optimum charging-discharging schedule of the battery by taking into account such factors.